QE on the D600 is higher than 6D as well. Not a lot though, 53% versus 50% for 6D but read noise is much lower in 6D,

Read noise cannot be fully characterized by a single number, as it varies across the ISO range. If you simply take the values from sensorgen.info and compare them directly, it likely will not give you the comparison that you seek, because the numbers often correspond to different ISO settings.

It is best to go back to the source of their data, DxOmark, and look at the dynamic-range plots. These give a good basis for read-noise comparison between cameras which have similar QE (i.e., within 10% or so), same sensor size and similar pixel count. For the 6D and D600 comparison, it's clear that a strong ISO-dependence exists. Below ISO 1600, the D600 has lower read noise, and substantially so near base ISO. They are essentially tied at ISO 1600, and above that, the 6D continues to show some advantage of optimization in its analog signal chain, whereas the D600 is simply using numerical scaling of the digital data.

As a side note, because of the D600's ISO implementation above ISO 1600 (it is purely numerical scaling of the ISO 1600 raw data), I do not recommend use of ISO settings above 1600 when shooting RAW with the D600; this also applies to the D800/E. There is nothing to gain but missing codes in the RAW data, and you are reducing your dynamic range.

Another important noise source for long exposures, which unfortunately is not often tested and reported, is dark current noise. This can easily be more significant than the read noise measured at short exposure times. It is strongly dependent on temperature, and also can vary significantly between production batches of the same sensor design; it is even sometimes seen to vary across an individual sensor.

full well depth is much the same for either and much larger than for D800.

The FWC and QE figures from sensorgen appear to be accurate, but keep in mind that the FWC figure is per sensel. It is also important to consider full-well capacity on an equal-image-area basis; in that case, the D800 and 6D/D600 only differ by about 10%.

I notice my D800E picks up the subtle and hard to record night glows of ionised gases in the atmosphere which are subtle reds, magentas and greens very well that I don't see in many Canon images. Again I haven't seen virtually 1 well done 6D image so hard to conclude but the D600 is likely to be a bit more sensitive to light than 6D.

It is worth considering that the various types of sky glow, be they from artificial sources or natural ionization, are also noise sources. Often, the noise that they contribute to the image is more significant than the camera's read noise and dark-current noise.

Dynamic Range should be similar at the higher ISOs you'll be using but better in the D600 at lower ISOs.